Chemistry · Lesson

Elementary steps, intermediates and reaction mechanisms

Check how proposed elementary steps add to an overall reaction. Identify an intermediate and explain why a plausible rate law does not uniquely prove a mechanism.

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By the end, you can

Cancel intermediates in a consistent step ledger.
Distinguish an elementary step law from an overall empirical rate law.
State assumptions when deriving an approximate rate from a slow first step.

A mechanism proposes a sequence

A balanced overall equation records net chemical change. A mechanism proposes elementary steps that could produce that change. Each step must conserve the relevant atoms and charge, and the complete step sum must reproduce the net equation. A plausible list is a hypothesis to test, not a consequence of balancing alone.

For an original abstract example, let A and B carry distinct conserved building blocks; I and P each contain both blocks and all species are neutral. The proposed steps A + B → I and I → P therefore conserve this stipulated composition. These symbols teach bookkeeping rather than identify real compounds.

An intermediate forms and is then consumed

Adding the two steps cancels I, giving A + B → P. I is an intermediate because it appears as a product of one step and a reactant of another while disappearing from the net equation. Its absence from that equation does not mean it is never present during the process.

A transition-state configuration is different from an intermediate species. In a qualitative energy profile, an intermediate corresponds to a local minimum between steps, whereas a barrier’s transition-state region lies near a maximum. Whether an intermediate can be detected depends on its lifetime, amount and the measurement.

Cancel a proposed intermediate in the step sum

Ledger row	Proposed change	Role of I	What the row establishes
Step 1	A + B → I	Produced	A proposed elementary encounter
Step 2	I → P	Consumed	A proposed conversion of the intermediate
Sum	A + B → P	Cancels	The net equation, with the stated composition

Original abstract mechanism ledger. A and B carry distinct conserved building blocks; neutral I and P each contain both. This makes the stipulated steps composition-consistent without asserting real compounds or a proven mechanism.

A step law needs the elementary-step assumption

For a simple elementary bimolecular step A + B → I under an appropriate mass-action regime, its forward rate can be k₁[A][B]. This concentration dependence is tied to the stated elementary event. Copying coefficients from an arbitrary overall equation does not establish its observed rate law.

If this first step is slow, the subsequent conversion I → P is sufficiently fast, reverse reactions are negligible and I does not build up significantly, product appearance can approximately follow k₁[A][B]. These assumptions connect the step rate to the observed overall rate; they are part of the claim.

Consistency is necessary, but not unique proof

A proposed mechanism should reproduce both the net equation and the observations under their conditions. A matching rate law can support it while leaving alternatives possible. Detecting intermediates, isotope effects or other independent evidence can add discrimination.

If a reversible step precedes a slower step, eliminating its intermediate concentration may require a justified pre-equilibrium or steady-state approximation. The highest absolute peak on a drawing alone does not select a universal controlling step; barriers must be referenced to the relevant states and the coupled kinetics considered.

Worked examples

Sum a composition-consistent mechanism

Use the abstract two-step mechanism A + B → I; I → P, with the stipulated conserved building blocks. Find the net equation and identify the intermediate.

Add both reactant lists and both product lists.
I appears once on each side and cancels.
The remaining net change is A + B → P.

Net A + B → P; I is the proposed intermediate. This bookkeeping does not independently prove the mechanism.

Try it yourself

What is the net equation for the stated A + B → I; I → P steps?
Show the answer and reasoning
A + B → P.
Cancel the same I on both sides of the summed ledger.
If I cancels from the net equation, does that prove it is never present?
Show the answer and reasoning
No.
It can form and be consumed during the proposed sequence.
When is k₁[A][B] justified for the first proposed step?
Show the answer and reasoning
When it is an appropriate elementary bimolecular mass-action step.
The elementary-step assumption differs from merely writing a net equation.
Does one matching empirical rate law uniquely establish the mechanism?
Show the answer and reasoning
No.
Different mechanisms can share that concentration dependence; further evidence can distinguish them.

What to carry forward

A mechanism must preserve composition and reproduce the net change.
Distinguish intermediates from transition-state configurations.
Keep elementary-step assumptions and approximations explicit.
A compatible rate law supports a proposal without uniquely proving it.

References

Teaching resources and reference material supporting this lesson’s explanations and examples.